For example, in a chart recorder with a so-called thermal comb whose heating elements, which are arranged along a straight line and fixed, are activated by the measurement signal and produce a measurement curve on heat-sensitive recording paper used as a recording medium, the curve portions currently being formed are covered by the thermal comb and are therefore invisible.
Also, for design or aesthetic reasons, the housing of a chart recorder may be so designed that the curve portions currently being formed disappear behind a covering and are therefore invisible.
The design and use of thermal combs in chart recorders are also described in U.S. Pat. No. 3,971,041.
Such a chart recorder with a thermal comb is, for example, the recorder being produced and sold by the applicant under the name of "MEGA-LOG", in which one to six measuring channels are provided, one for each of the measurement signals to be recorded. The thermal comb consists of a plurality of point heat sources which are arranged side by side along the width of the recording paper and are constantly in contact with the latter. To produce the dot sequence associated with a measuring channel, a short current pulse is applied to the point heat sources for each dot.
Jap. Publctn. JP-A 62-204 972 discloses a typewriter wherein the characters currently being written on the keyboard and thus printed on the typewriting paper are also made visible line by line on an electronic alphanumeric display to immediately display the characters being hidden by the printing mechanism or other parts of the typewriter.
Thus, this facility is only designed to additionally visualize the alphanumeric characters deliberately and intentionally produced by the writer, i.e., characters whose information content is already known to the writer.
By contrast, those portions of the above-mentioned dot sequences of chart recorders which are currently being formed contain nonalphanumeric information which is not yet known to the viewer and is frequently of the greatest interest to him, e.g., information on whether and at what rate measurement signal changes, i.e., what its trend is at the measurement time and in the vicinity thereof. Because of the covering, however, this possibly very important information is accessible only with a delay.
The problem underlying the invention is to eliminate this disadvantage.